Oakland County prosecutors have dropped 16 drug cases in recent months — including one involving a large-scale marijuana bust — after an investigation determined a deputy on the county’s narcotics enforcement team falsified a search warrant and lied under oath.

County Prosecutor Jessica Cooper said she learned in September that Marc Ferguson, now fired, opened a shipping container at YRC Freight in Pontiac without a search warrant in June 2011. He discovered 78 pounds of marijuana with a street value of $300,000.

Ferguson resealed the container and sought a search warrant from a Pontiac magistrate, signing a sworn affidavit under oath that asked for permission to open the container, Cooper said. Police arrested Anastacio Payan of California when he tried to claim the container, and in a court hearing the next month, Ferguson denied on the witness stand that he opened the container without a warrant.

Under state and federal laws, police officers must go before a magistrate or judge and present probable cause before they are allowed to search things such as packages, cars, homes or businesses.

Drug trafficking charges against Payan were dropped Sept. 24. Prosecutors then sifted through 100 pending drug cases involving Ferguson and dismissed 15 other cases in which he was central to the investigation.

“It was the necessary, ethical thing to do,” Cooper said.

Ferguson was fired in December, following an internal investigation by the Oakland County Sheriff’s Office. He has filed a grievance to get his job back.

Ferguson could not be reached for comment. Union officials representing him declined to comment.

Ferguson has not been criminally charged. Oakland County Undersheriff Michael McCabe said the department didn’t seek criminal charges for fear that a possible acquittal would allow Ferguson to get his job back through arbitration.

“Juries have a difficult time convicting police officers,” McCabe said, noting that there have been numerous cases in which officers who beat criminal charges were reinstated. “And then we would be stuck with him. We can’t take the chance that the jury will acquit.”

The dishonest testimony came to light when an assistant prosecutor who was preparing for the marijuana case last September learned of the false affidavit after interviewing witnesses and notified Cooper, she said. Cooper then alerted the Sheriff’s Office.

“Once we were made aware, we started an internal investigation, and he was removed from the narcotics team,” McCabe said. “After an eight-week investigation, including interviewing the other officers in the crew, we concluded that he needed to be terminated and could no longer wear a badge.”

Ferguson, 47, was a Pontiac police officer for 24 years before he was hired by the Sheriff’s Office when that agency took over Pontiac’s policing responsibilities in 2011.

He spent 13 years in narcotics enforcement and has previously testified that he investigated 10,000 drug cases.

In a 2011 drug case, two of seven charges were dropped against career criminal Marcus Kelley after prosecutors said they learned Ferguson conducted a raid on Kelley’s Pontiac home before a warrant was signed. Kelley was convicted of the other five charges.

Kelley’s attorney Richard Taylor said Monday that defense attorneys all over the county are reviewing their cases to see whether Ferguson had any involvement — “especially if someone was found guilty,” he said.

In the shipping container case, a worker at the YRC Freight on Joslyn Road in Pontiac alerted Ferguson on June 22, 2011, that a suspicious-looking container had arrived.

Large drug shipments sometimes arrive in crates that look like they hold construction materials. After Ferguson got there, he summoned other narcotics investigators.

Ferguson would later write in his report that the container reeked of marijuana, although none of the other investigators noted that. Investigators said they believe that is when Ferguson opened the container and then resealed it, McCabe said.

When Payan arrived in a Ford Explorer, Ferguson posed as a worker at the yard and helped load the crate into the vehicle. When Payan drove away, officers arrested him.

At Payan’s preliminary exam on July 6, 2011, his attorney James Galen asked Ferguson on the witness stand, “Prior to turning the box over to Mr. Payan, did you or anyone, to your knowledge, pick those locks and look inside that box?”

Ferguson replied, “No.”

Galen said Monday that he was pleased the truth came out.

“I knew he was dirty, and I tore him apart in the preliminary exam,” Galen said. “I felt there was police misconduct, and I was trying everything to pry it out of him.”

The Payan case wasn’t the first time Ferguson had investigated a shipping box at YRC.

In January 2011, just months before, the same employee called Ferguson when a construction crate arrived. It contained marijuana.

Following an investigation, a Dryden man and a Colorado woman were arrested and convicted.

Legal experts predicted Ferguson’s prior cases will now be under review.

“Everything he did is now going to be subject to scrutiny, and at great administrative costs,” said Peter Henning, a constitutional law professor at Wayne State University. “Sure, getting a warrant is a hassle, it slows down police work, but you don’t cut corners. It’s one of the reasons we had a revolution. It is at the core of the Constitution.”